Automatic coagulation instruments for measuring plasma clotting times are well known in the art. For example, U.S. Pat. No. 3,607,099 discloses an instrument capable of doing the prothrombin time (PT) test serially on a plurality of plasma samples and printing the test results on a paper tape. U.S. Pat. No. 3,767,364 discloses an instrument having two reagent reservoirs from which reagents are pipetted into plasma samples. The use of two reagents facilitated testing for the activated partial thromboplastin time (APTT) of a plasma sample. U.S. Pat. No. 3,969,079 discloses an instrument that employs a dual channel cuvette disk that permits a prothrombin time test to be performed simultaneously on two plasma samples, or an APTT test to be performed simultaneously on two plasma samples, or a PT test on one plasma sample simultaneously with an APTT test on a second plasma sample. Each of the instruments disclosed in the aforementioned patents contemplated performing the same tests over and over again on plasma samples that were serially advanced to reagent dispensers and then to a test station where clot formation was determined in a clot detection circuit employing photo-optical means. When changing from one test mode to another in the instrument of U.S. Pat. No. 3,969,079, it would be necessary to replace at least one of the reagent reservoirs to go from a PT test mode to an APTT test mode, or to a PT/APTT test mode.
At the present time the APTT test represents approximately 40% of the total test load of a typical clinical laboratory. The PT test represents 50-55% of the test load, with the remainder being miscellaneous other tests. Usually, if a test is requested to determine the APTT of a patient's plasma, a test will also be requested to determine the PT as well. On the other hand, if a PT test is requested, an APTT test may not be requested. Thus, it is clear that a pattern of test requests may be random, and may not be met by instruments that are most suitable for repetitive performance of the same type test.